1. Field of the Invention
The present invention relates generally to electrical devices and to the assembly of electrical devices. More particularly, the present invention relates to the assembly of radio frequency identification (RFID) interposers and/or devices.
2. Description of the Related Art
Pick and place techniques are often used to assemble electrical devices. Pick and place techniques typically involve complex robotic components and control systems that handle only one die at a time. Such techniques may utilize a manipulator, such as a robotic arm, to remove integrated circuit (IC) chips, or dies, from a wafer of IC chips and place them on a chip carrier, a transport, or directly to a substrate. If not directly mounted, the chips are subsequently mounted onto a substrate with other electrical components, such as antennas, capacitors, resistors, and inductors to form an electrical device.
One type of electrical device that may be assembled using pick and place techniques is a radio frequency identification (RFID) transponder. RFID inlays, tags, and labels (collectively referred to herein as “transponders”) are widely used to associate an object with an identification code. Inlays (or inlay transponders) are identification transponders that typically have a substantially flat shape. The antenna for an inlay transponder may be in the form of a conductive trace deposited on a non-conductive support. The antenna has a suitable shape, such as a flat coil or other geometric shape. Leads for the antenna are also deposited, with non-conductive layers interposed as necessary. Memory and any control functions are provided by a chip mounted on the support and operatively connected through the leads to the antenna. An RFID inlay may be joined or laminated to selected label or tag materials made of films, papers, laminations of films and papers, or other flexible sheet materials suitable for a particular end use. The resulting RFID label stock or RFID tag stock may then be overprinted with text and/or graphics, die-cut into specific shapes and sizes into rolls of continuous labels, or sheets of single or multiple labels, or rolls or sheets of tags.
In many RFID applications, it is desirable to reduce the size of the electrical components as small as possible. In order to interconnect very small chips with antennas in RFID inlays, it is known to use a structure variously called “interposers”, “straps”, and “carriers” to facilitate inlay manufacture. Interposers include conductive leads or pads that are electrically coupled to the contact pads of the chips for coupling to the antennas. These pads generally provide a larger effective electrical contact area than ICs precisely aligned for direct placement without an interposer. The larger area reduces the accuracy required for placement of ICs during manufacture while still providing effective electrical connection. IC placement and mounting are serious limitations for high-speed manufacture. The prior art discloses a variety of RFID interposer or strap structures, typically using a flexible substrate that carries the interposer's contact pads or leads.
As noted above, RFID transponders include both integrated circuits and antennas for providing radio frequency identification functionality. Interposers, on the other hand, include the integrated circuits but must be coupled to antennas in order to form complete RFID transponders. As used in the present patent application the term “device” refers both to an RFID transponder, and to an interposer that is intended to be incorporated in an RFID transponder.
RFID devices generally have a combination of antennas and analog and/or digital electronics, which may include for example communications electronics, data memory, and control logic. For example, RFID tags are used in conjunction with security-locks in cars, for access control to buildings, and for tracking inventory and parcels. Some examples of RFID tags and labels appear in U.S. Pat. Nos. 6,107,920, 6,206,292, and 6,262,292, all of which are hereby incorporated by reference in their entireties.
An RFID device may be affixed to an item whose presence is to be detected and/or monitored. The presence of an RFID device, and therefore the presence of the item to which the device is affixed, may be checked and monitored by devices known as “readers.”
Typically, RFID devices are produced by patterning, etching or printing a conductor on a dielectric layer and coupling the conductor to a chip. As mentioned, pick and place techniques are often used for positioning a chip on the patterned conductor. Alternatively, a web containing a plurality of chips may be laminated to a web of printed conductor material. An example of such a process is disclosed in commonly assigned U.S. patent application Ser. No. 10/805,938, filed on Mar. 22, 2004.
The chips may be coupled to the conductor by any of a variety of suitable connecting materials and/or methods, such as, for example, by use of a conductive or non-conductive adhesive, by use of thermoplastic bonding materials, by use of conductive inks, by use of welding and/or soldering, or by electroplating. Typically, the material used for mechanically and/or electrically coupling the chip to the conductor requires heat and/or pressure to form a final interconnect—a process, in the case of adhesives, known as curing. Conventional thermocompressive bonding methods typically use some form of press for directing pressure and heat, via conduction or convection, to an RFID device assembly or web of RFID device assemblies. For example, pressure and heat may be applied by compressing the RFID device assembly or web of RFID device assemblies between a pair of heating plates, and relying on conduction through the various media, including chip and antenna, to heat the connecting material. Alternatively, one of the heating plates may be equipped with pins for selectively applying pressure and/or heat to certain areas (e.g. only the chips), and again relying on conduction to heat the connecting material. Alternatively, and especially in the case of solder, an oven may be used wherein the whole assembly is held at elevated temperature and via convection the solder reflows. In the latter case, pressure may not be applied to the device.
However, conventional RFID inlay or interposer manufacturing techniques using flip-chip assembly are generally unable to produce devices at a rate fast enough to satisfy demand. Considerable effort is required to accurately align the chips with the antenna structures often limiting the rate at which devices may be produced. Further, conventional flip-chip manufacturing methods produce a device of varying thickness by placing the chip atop a substrate or other surface. Thus, the sides of the chips are generally exposed, making the device more vulnerable to damage. An ideal RFID tag or label would be very thin and also have a substantially uniform thickness. One of the problems with conventional RFID inlay and interposer manufacturing techniques, as well as the RFID devices themselves, is that the chip thickness is greater than the substrate thickness, frequently by a large factor. Placing the chip onto a film or web substrate creates a bump wherever the chip is located. Such a bump presents problems for printing equipment that may subsequently print text or graphics onto the label facestock. In the case of RFID labels or other devices that typically may be printed upon, the uneven surface of the device may interfere with the printing process causing distortion and/or printing errors.
Therefore, it is desirable to provide a high-speed method of manufacturing electrical devices wherein the finished device has a relatively flat profile.
From the foregoing it will be seen there is room for improvement of RFID transponders and manufacturing processes relating thereto.